The use of remote control to effect a desired operation of various systems and devices is relatively widespread. For example, inaccessible or inconveniently located systems are particularly suited For remote control. Similarly, when a number of widely dispersed systems are involved, remote control can be used to provide the desired operation of each system from a single control point. In either case, control is easily achieved with a minimum of operator time, thus reducing system operating costs.
One way in which reliable remote control has been achieved is with the aid of communication systems. Such communication systems produce a suitably encoded signal, usually by modulating one or more tones of specified frequency and duration onto an appropriate radio frequency (RF) carrier wave, and transmitting the signal to a remote system to be controlled. The encoded signal is received by the remote system and demodulated, with the resultant tones being applied to a decoder. The decoder produces an output signal that effects a change in the operation of the remote system only upon receipt of one or more tones having the specified frequency and duration. Otherwise, the operational status of the remote system remains unchanged.
One particular type of communication system that has found widespread commercial acceptance in the logging industry employs an encoded signal commonly identified as a "two-tone sequential" signal. The encoded signal is used to effect the remote actuation of a whistle or other signaling device to indicate to personnel information concerning the operation of logging equipment. The assignee of the present application has a number of patents related to two-tone sequential systems. Two of such patents include: U.S. Pat. No. 27,044, directed toward a "Two-Tone Remote Control System," issued Feb. 2, 1971 to Rothenbuhler et al. and U.S. Pat. No. 4,197,525, directed toward a "Tone Decoder," issued Apr. 8, 1980 to Biery, Jr. et al.
The operation of the systems disclosed in the above-referenced patents may be described briefly as follows. A first tone of limited duration and of a specified frequency is modulated upon an RF carrier by a transmitter. Immediately following the termination of the first tone, a second tone having a specified frequency that is different from the frequency of the first tone is modulated upon the RF carrier. The duration of the second tone is equal to the desired actuation time of the remote signaling device and the frequency of both tones is typically in the audible frequency range.
At the location of the whistle, signals having the frequency of the RF carrier are received and demodulated. The demodulated signals are then applied to a decoder. The function of the decoder is to first identify the occurrence of a demodulated signal having a frequency corresponding to the nominal frequency of the first tone. In that event, the decoder continues to monitor the demodulated signal for the occurrence of a signal having a frequency corresponding to the nominal frequency of the second tone. The decoder provides an output signal to the whistle when the frequency of the demodulated signal corresponds to that of the second tone, but only if such a demodulated signal is detected within a predetermined period of time following detection of the first tone. Such a "two-tone sequential" system provides reliable, remote actuation of the whistle, even in the presence of encoded signals used in other communication systems that may have the same RF carrier modulated with a single tone at the nominal frequency of either the first or the second tone.
Another patent issued to the assignee of the present invention that also relates to two-tone sequential decoding systems is U.S. Pat. No. 5,007,069, titled "Decoding of Signals Using Cophase and Differential Signal Detection," and issued Apr. 9, 1991 to VanEss. VanEss discloses a decoder for producing an output signal when a receiver receives signals that fall within a predetermined pass band, i.e., a range of frequencies bounded by upper and lower frequencies f.sub.1 and f.sub.2. The decoder includes a tone detector having a cophase tone detection circuit and a differentiating tone detection circuit. The combination of cophase and differentiating tone detection enhances both the speed and accuracy of decoder. With reference to FIG. 3 of VanEss, a cophase reference signal R, having a frequency f.sub.r corresponding to the tone to be detected, is produced. Both phases of the cophase reference signal are mixed with an input signal received by the receiver having a frequency f.sub.i, with the resultant signals containing information regarding a frequency difference .DELTA.f that is equal to the .vertline.f.sub.i -f.sub.r .vertline.. This output, in the form of triangular waveforms, is supplied separately to both the cophase and differentiating tone detection circuits.
The cophase detection circuit determines whether .DELTA.f is less than a predetermined first cutoff frequency f.sub.c1. This step is accomplished by determining if the triangular input waveform is present within a window covering the frequency range zero to f.sub.c1 Hz. The differentiating tone detection circuit produces a derivative-based confirmation of .DELTA.f. An output is produced from the circuit if the derivative of .DELTA.f is less than a predetermined second cutoff frequency f.sub.c2, or if .DELTA.f is greater than f.sub.c1. A single tone detector output is then produced if the difference .DELTA.f between the frequency f.sub.i of the input signal and the frequency f.sub.r of each phase of the cophase reference signal is less than both cutoff frequencies f.sub.c1 and f.sub.c2.
As will be appreciated from the preceding discussion, much effort has been expended to provide a tone decoder that is able to detect tones with a high level of precision. More particularly, while the use of "two-tone sequential" encoding is designed to minimize the unintended actuation of a remote system or device, the effectiveness of any encoding technique is proportional to the ability of the decoder to precisely identify one or more tones in sequence. Further, it is typically advantageous that the decoder respond quickly to encoded information to avoid undue delay in the control of the remote system. It would be desirable to provide a decoder that is able to both quickly and accurately identify one or more tones encoded upon a carrier wave to accomplish remote control objectives with even greater speed and accuracy.